Distribuição espaço-temporal de Scolelepsis cf. squamata (Muller, 1808) (Polychaeta: Spionidae) como ferramenta para estudos da dinâmica sedimentar costeira

Abstract

A population of the spionid polychaete Scolelepis cf. squamata was studied on Dois Rios beach, an embayed oceanic beach located at Ilha Grande (a coastal island), at the southeastern Brazilian coast. The relationship between beach morphodynamics and the spatial-temporal distribution of this species along and across the beach was evaluated in relation to density and body size variables. Sampling was carried out during spring low tides of May/2001, July/2001, November/2001 and January/2002. Eight transects with different hydrodynamical influences were chosen along the entire beach. The intertidal beach was divided into three across-shore zones, where three replicate samples were randomly collected from each zone at each transect. Differences in S. squamata longshore distribution occurred mainly due to beach morphodynamics, where species densities increased from erosional (low tide terrace) to depositional (reflective) beach states and species body size increased from depositional to erosional ones. However, exceptions that could not be explained by the morphodynamic state per se, seems to be related to hydrodynamic patterns. The association between intermediate to large S. squamata body sizes and coarser skewed sands, shows the great importance of passive transport of individuals in determining species distribution. A correlation between sediment size and S. squamata distribution were not found, although a clear relationship between species distribution and marine sediment transport were found. S. squamata densities were lower during high hydrodynamic periods due to their weaker resistance to transport by wave action. In spite of the high hydrodynamics, smallest S. squamata body sizes were found in this period, that coincided with a species recruitment. The close association observed among S. squamata spatialtemporal distribution, morphodynamic beach state and sediment transport, supports the hypothesis that this species density and body size distribution might be used as a tool for correlating beach morphodynamics, hydrodynamics and sediment transport in oceanic sandy beaches.